FIG. 1 shows a partial sectional view of a photodiode for a comparative explanation of the present invention. The illustrated photodiode has a p-type region 1 for accumulating photocharges in a semiconductor layer made of silicon (not shown), an insulator layer 5 being in contact with the semiconductor layer, and an n+-type region 2 formed in the section from the interface between the semiconductor layer and the insulator layer 5 to the p-type region. The n+-type region 2 has a region which is in contact with the p-type region 1 and which forms a depleted layer, and a non-depleted region 9 which is in contact with the insulator layer 5. The interface between the non-depleted region 9 and the insulator layer 5 has roughness 11. The roughness 11 of the interface between the semiconductor layer and the insulator layer in a region having a length of 1 μm in the direction parallel to the interface have a height of approximately 1 nm, as can be confirmed in FIG. 3 which shows an atomic force microscope image of the interface and a profile of the roughness. The thickness 10 of the non-depleted region 9 of the n+-type region 2 is made to be considerably large, e.g. 100 nm, so as to suppress a dark current resulting from a carrier-producing current due to the interface state.
As in the case of the photodiode shown in FIG. 1, if the thickness 10 of the non-depleted region 9 of the n+-type region is 200 nm and hence greater than the penetration depth 12 of the ultraviolet light, the electric field for the drift transport of photocharges toward the p-type region 1 in the non-depleted region 9 of the n+-type region 2 is insufficient for efficiently transporting the photocharges generated by the ultraviolet light to the n-type region, and the photocharges annihilate due to recombination. Thus, sensitivity to the ultraviolet light is low.
On the other hand, in the case of the photodiode shown in FIG. 2, where the thickness 10 of the non-depleted region 9 of the n+-type region 2 is on average smaller than the penetration depth 12 of the ultraviolet light, a section in which the thickness is smaller than the roughness 11 of the interface locally occurs. In such a local section where the thickness 10 of the non-depleted region 9 of the n+-type region 2 is smaller than the roughness 11 of the interface, the generation and recombination of electric charges due to the interface state frequently occurs, producing a high dark current. Furthermore, the annihilation of photocharges due to the recombination particularly lowers the sensitivity to the ultraviolet light in the wavelength range of 200-320 nm, which has a short penetration depth, among the light in the wavelength range of 200-1100 nm.
Furthermore, due to the interface state or a local depletion of the n+-type region 2 caused by the fixed charges in the insulator layer 5 resulting from a long-term irradiation with the ultraviolet light, the dark current increases and the sensitivity to the ultra violet light fluctuates. Thus, the dark current and the sensitivity have been unstable against irradiation with the ultraviolet light.